1. Field of the Invention
The present invention relates to a recording and reproducing apparatus, and more particularly, to a recording and reproducing apparatus in which page data is multiply-recorded with a hologram by irradiating a holographic recording medium with an information light beam and a reference light beam at the same time.
2. Description of the Background Art
According to the recording and reproducing apparatus using the hologram, two-dimensional page data is recorded by irradiating a certain position of the holographic material layer of the holographic recording medium with an information light beam corresponding to the two-dimensional page data and a reference light beam at the same time.
The two-dimensional page data is recorded as an interference pattern of the two light beams (information light beam and reference light beam).
Meanwhile, page data recorded on the medium is reproduced by irradiating the medium with only the reference light beam and detecting its reflected light beam or a transmitted light beam (referred to as a reproducing light beam also) in a two-dimensional image device (CCD, for example).
According to the holographic recording method, since the page data is recorded as the interference pattern, a plurality of page data can be recorded in the same recording region by multiple recording. As a method of the multiple recording, various kinds of methods such as angular multiplexing, frequency multiplexing, phase multiplexing, and shift multiplexing have been proposed
In addition, as one aspect of the multiple holographic recording, a method using a speckle pattern is proposed.
The speckle pattern is a pattern formed on a screen when a coherent light beam is passed through a diffusion board as shown in FIG. 11 (refer to Applied Optics I, written by Tadao Tsuruta, Baifukan Inc., July in 1990, p. 244).
The diffusion board is a transparent board comprising fine indented patterns on its surface and it is an optical element which varies the phase of the coherent light beam when it transmits the light beam.
How the speckle pattern is distributed on the screen depends on the random nature of the indented pattern on the surface of the diffusion board. According to the holographic recording, the diffusion board is irradiated with the reference light beam and the speckle pattern of the reference light beam is used in recording and reproducing processes. The speckle pattern is used to improve angular selectivity or recording multiplicity when information is recorded by multiple recording.
FIG. 12 is a schematic explanatory diagram showing angular multiple recording in conventional holographic recording.
Here, only a constitution in the vicinity of a holographic recording medium of a holographic recording and reproducing apparatus is shown.
A light beam emitted from a light source is divided into two by a beam splitter (not shown), and one laser beam (referred to as an information light beam or an object light beam) is applied to a spatial light modulator (SLM) and passed through an objective lens and applied to the medium in almost the perpendicular direction.
The other light beam (referred to as a reference light beam) is passed through the mirrors and a diffusion board and applied to the medium at an angle θ.
The information light beam and the reference light beam are applied to the medium at the same time and different information can be recorded in the same position of the medium by varying the irradiation angle θ of the reference light beam.
Referring to FIG. 12, although the angular multiple recording can be performed by varying the irradiation angle θ of the reference light beam without the diffusion board, when the reference light beam is patterned with speckles by the diffusion board, angular selectivity can be improved. In other words, an amount of information which can be multiplied in the same position can be increased.
Referring to FIG. 12, in order to implement the angular multiplexing, a position or an angle of the mirror arranged in the way of a light path of the reference light beam has been adjusted with an actuator conventionally.
FIG. 13 is a schematic explanatory diagram when multiple recording is performed using both shift multiplexing and speckle multiplexing in the conventional holographic recording.
Points in which an information light beam and a reference light beam are used and a SLM and an objective lens are provided are the same as in FIG. 12. In addition, a mechanism to move a medium in the lateral direction in the drawing with respect to the SLM and the objective lens is provided to perform the shift multiplexing in FIG. 13.
In addition, in order to perform speckle multiplexing, there is provided a mechanism (linear moving mechanism using a stepping motor or a piezo element, for example) to move a diffusion board in a direction vertical to an optical axis of the reference light beam as shown in FIG. 13.
In this case, the multiple recording can be performed in a certain recording position of the medium by moving the diffusion board linearly little at a time, which is called the speckle multiplexing.
In addition, after the recording by the speckle multiplexing in the same recording position, recording is performed in the position which partially overlaps with the position in which the recording was performed by the speckle multiplexing, by slightly moving the position of the medium in the lateral direction, which is called the shift multiple recording.
Then, when the diffusion board is moved and the recording is further performed by the speckle multiplexing in the position after the medium was slightly moved, since the speckle multiplexing is performed in addition to the shift multiplexing, recording multiplicity of information can be further improved.
As one of documents disclosing multiple recording, there is known Japanese Unexamined Patent Publication No. 2002-216359.
However, according to the conventional recording and reproducing apparatus, a problem lies in reproducibility of the position of the diffusion board. For example, according to the angular multiplexing, although the information can be recorded by multiple recording by only varying the irradiation angle θ of the reference light beam slightly, when certain recorded page data is to be reproduced contrary, an irradiation angle θ1 at the time of recording has to be equal to an irradiation angle θ2 at the time of reproducing precisely. Even when the θ2 is different from the θ1 slightly, a reproducing error could be generated.
In addition, as shown in FIG. 13, when the shift multiplexing and the speckle multiplexing are combined, moving control of the medium and moving control of the diffusion board have to be performed with high precision. Even when the position of the medium can be correctly determined, if the reproducing process is performed in a state the position of the diffusion board is slightly shifted from the position at the time of recording, a reproducing error is generated.
That is, the moving control and positioning of the diffusion board has to be performed with high precision so that the position at the time of recording may be the same as the position at the time of reproducing, which needs a high degree of positioning control.
In addition, when the two movement controls are to be performed with high precision, a control circuit size becomes large, costs becomes high and it takes time to control the movement.
However, when a data transfer speed at the time of recording and reproducing is to be improved, it is necessary to shorten a time for moving the position of the diffusion board or the medium, so that it is very difficult to implement high reproducibility of the diffusion board and the like.
Especially, when the recording and reproducing are performed using the speckle pattern to improve the recording multiplicity, since it is necessary to provide the diffusion board in the way of a light path of the reference light beam, it is essential to implement more correct position reproducibility of the diffusion board in order to ensure improvement of reliability of recording and reproducing or promptness of the reproducing process in addition to improvement of recording capacity.